Support running powertest locally.

• Move most of this embedded shell into powertest.py (or its equivalence in resmoke.py). The new file should be mostly runnable locally. Consider rewriting the code in Python so it's locally testable, especially the report generation and Windows override parts.
  For the Windows overrides, it may be easier to run the task in evergreen and print out the existing expansions than to reason about what each one means given the large quantity of them.
• Move the per-task powercycle expansions in evergreen.yml to a separate YAML file.
• We should now be able to simplify the powertest.py invocation to just the IP address, the OS, and the task name with an optional expansions file if absolutely necessary. This allows the same command to be used locally and in evergreen.

Finally we want to document the new arguments and the prerequisites for running powercycle locally, which is mostly just calling evergreen host create ...